This invention relates to the field of packet-switching networks and, more specifically, data broadcast networks. The Internet is the global packet-switching network and the digital television is a data broadcast network utilizing packet multiplexing.
The guaranteed error-free data delivery over the Internet is provided by an acknowledgement mechanism of TCP protocol. According to the protocol, the sender retransmits a packet if the receiver does not acknowledge the reception of the error-free packet. The positive acknowledgement or ACK provides for both packets recovery and congestion control—the sender slows down if ACKs are delayed. A “negative acknowledgement” or NAK, which is a request for retransmission of a lost or corrupted packet, is used for packets recovery only.
Cable, satellite and terrestrial television networks are using digital compression and packet multiplexing in order to increase the number of channels but as of today, they operate as one-way systems. (Though cable plant has a return channel for Internet access, video-on-demand and telephone over cable.) Accordingly, no retransmission requests are sent back to the sender; instead errors are handled by a forward error correction mechanism (FEC) which requires more redundant bits and does not provide a guaranteed error-free delivery.
Return channels may improve television in three ways providing content on demand, packets recovery and audience measurement.
While video is widely considered as error-tolerant, the very reason to pay for cable or satellite service is a better picture quality, which in turn is a result of lower error rates. Using a return channel over DSL for packets recovery may dramatically improve the quality of terrestrial broadcast television, which is free of charge.
In the television industry the audience size is determined by Nielsen Ratings provided by Nielsen Media Research. Since television as a business makes money by selling audiences to advertisers, the Nielsen Ratings are used for determining the cost of advertising spots in television programs. The ratings are gathered either by using surveys, where a viewer keeps a diary of watched programming in exchange for a fee, or by using a meter device connected in selected homes to TV set for gathering the viewing habits and transmitting the information to Nielsen through a phone line. A criticism of the system is that it fails the most important criteria of a sample: it is not random in the statistical sense because only those viewers who accept diaries or meters are included in the sample.
In a data broadcast system, each receiving device represents a user and a group of devices, which simultaneously receive or download a particular content, represents the audience of the content. With a return channel being provided the audience measurement can be automated and improved.
There is however a difference between broadcast systems and conventional computer networks. On the Internet, a separate copy of content is served to each client computer even when thousands of clients request the same content. While this delivery model wastes the network resources and creates congestion, the audience is measured simply by the number of “hits”. In a data broadcast system, all receiving devices receive the same signal and therefore the same “copy” of content—and the same corrupted packet as well. So should each receiver send a retransmission request whenever it receives a corrupted packet? The answer seems to be negative because there might be thousands and even millions of recipients of the same content and just one request would be enough to recover the packet, while multiple requests would reduce the bandwidth savings of the “one copy to all” delivery. On the other hand, it is impossible to designate a particular receiver as a request sender because it is unknown in advance which receivers will make up an audience of a particular content.
This invention is a method and system for packets recovery in data broadcast network with a return channel. The system operates so that not all but only a few receivers submit requests for retransmission of a corrupted packet no matter how many receivers have received the packet. The system operations include an audience measurement on the sender side by transmitting packets with wrong error-checking values and evaluating receivers' responses. While the efficient error handling improves the network performance, the audience measurement is important for network business as long as transmitted content is funded by advertisers.
An application of this technique for error handling and audience measurement is Broadcast Modem—an Internet access system disclosed in U.S. Pat. Nos. 6,698,023, 6,965,913 and 7,092,999 by the same applicant. The system delivers Internet content in two steps. First, a single copy of content is downloaded from its origin web server into a server at a broadcast center according to the standard Internet protocol. Then the copy is transmitted to client computers according to a data broadcast protocol so that all clients could download it simultaneously. Without any change to the existent Internet infrastructure, the two-step delivery reduces congestion on web sites and routers, provides users with virtually instant access to information of common interest and facilitates delivery of full-length DVD-quality video from Web sources.